Digital speckle photography technique evolves from the original speckle photography technique developed years before the invention of lasers. The technique involves exploiting the changing position of speckles, either naturally occurring or artificially seeded, to determine changes to an object as a result of deformation. Over the years, speckle photography has evolved into techniques such as laser speckle photography, electron speckle photography, white light speckle photography, and one-beam laser speckle interferometry.
Up until the advent and ubiquitous usage of digital camera, the process of generating useful information from a specklegram is nearly always done by using a laser beam. In the pointwise approach, a narrow laser beam is directed at a point of a double exposed specklegram, the resulting diffraction pattern consists of a circular halo within which a Young's fringe pattern is displayed representing the displacement vector experienced by the small cluster of speckles within the diameter of the laser beam. For the full field approach, an optical spatial filtering process is employed to display the displacement contours resolved along a particular direction with a sensitivity corresponding to the particular spatial frequency. In the early 1990s the process was digitalized. Examples of such digitizing methods are described in detail in Chen et al. (1) (Computer-aided Speckle Interferometry Using Spectral Amplitude Fringes, App. Opt., 1993, 32(2):225-236; incorporated by reference in its entirety) and Chen et al. (2) (Digital Speckle Displacement Using a Complex Spectrum Method, App. Opt., 1993, 32(11): 1839-1849; incorporated by reference in its entirety).
In the typical prior art full-field approach, a two-step Fourier transform (via FFT) algorithm is used, rendering the software computationally efficient. The software is often referred to as CASI (Computer Aided Speckle Interferometry). In CASI, the digital speckle patterns before and after deformation are divided into sub-images of a certain pixel array, for example, 32×32 pixels. The speckle patterns within the sub-images are “compared” with the corresponding one after the specimen is deformed, via a 2D Fourier transform process with a numerical multiplier. After another 2D Fourier transform operation an impulse function is obtained whose position vector is nothing but the displacement vector collectively experienced by the cluster of speckles within the sub-image. Thus by scanning through all the sub-images pairs, the displacement field of the entire area can be obtained. And the strain field can also be obtained using an appropriate strain-displacement relation.
Many attempts have been tried to transform the 2D speckle photography technique into a true three dimensional one with the capability of measuring internal strain field. For a transparent material, speckles can be seeded or embedded inside a 3D solid, and a beam or a sheet of light can be used to probe the interior strain at isolated points or planes. These approaches are tedious and time consuming and one can only probe a few planes. Furthermore, many important mechanics problems cannot be simulated by a transparent material.